IV-VI lead salt semiconductors have wide spread applications for devices such as mid-infrared lasers, detectors and thermal electrical cooling devices. Previously, these devices have been fabricated using either [100] or [111] substrates such as [100] IV-VI substrates, [111] BaF2 substrates and [111] Si substrates. Efforts have also been made to fabricate devices on III-V GaAs, GaSb substrates and II-VI CdTe, CdSe substrates.
In the area of lasers, much of the interest of recent world-wide explosion of mid-infrared (IR) laser diode research derives from the prospects for ultra-high-sensitivity chemical detection. The absorption spectra of trace gases are rich with features indicative of the molecular bonds present in the species. Large libraries of these spectra exist today which can be used for identifying unknown substances. For this reason, optical absorption spectroscopy has for decades been used in the identification of unknown substances by matching their distinct spectral absorption bands to the known library of spectra through a variety of “spectral fingerprinting” techniques.
The wide range of IR gas sensing applications includes pollution monitoring, detection of contraband (including chemical weapons, explosives, and drugs), toxic gas and chemical warning systems for buildings or military units, factory process control, automobile exhaust testing, and on-board exhaust analyzers, for example. Such gas sensing applications can be used to minimize vehicle pollution, perform safety and emissions monitoring for offshore, petrochemical and processing industries, and effectuate remote or personal-explosive hazard alarms.
Desired performance requirements of laser devices that are not currently available include tunable continuous wave (CW) operation at TE cooler temperature, spectral purity, and high output powers with good beam quality.
Currently, quantum cascade (QC) lasers, GaSb-based type II quantum well (QW) lasers, and IV-VI lead salts diode lasers are the leading approaches being pursued to meet gas sensing application needs. In the past decade, both QC and type II QW lasers have been proven successful. For example, optically pumped type II W-structure QW lasers have obtained continuous wave (CW) operation at 290 K using diamond-pressure-bond techniques. Up to the present, QC lasers have operated in CW mode above room temperature.
Among narrow gap semiconductors, IV-VI materials such as PbSe suppress Auger non-radiative loss (by more than an order of magnitude over the best III-V quantum wells), and have much lighter electron and hole masses that lead to further reduction of lasing thresholds. Previously, IV-VI materials have enabled lead salt lasers to set and maintain the records for maximum operating temperatures for both pulsed and CW operation among all mid-IR semiconductor diodes. They also provide advantages of easy current tuning, narrow linewidth, reproducibility and low cost.
For a number of years, lead salt diodes on [100] orientated substrates have been the only commercially available semiconductor mid-IR. However, their performance remains far from that desired because of their low operating temperatures and low efficiencies (single-mode output powers are typically≦1 mW, even at 77 K). There is also a tendency toward multi-mode operation and mode hopping. Lasing thresholds are significantly increased by the four-fold degeneracy of the L-valley conduction and valence band extrema. Quantum confinement does not lift the degeneracy in edge-emitting QW devices, since the four valleys remain symmetric for the [100] growth that must be employed to allow for the cleaving of laser cavities. This prevents the full exploitation of what is perhaps the greatest advantage of the IV-VI materials for high-temperature and long-wavelength operation, namely the threshold reduction that results from a low non-radiative recombination rate.
To solve some of the above-mentioned problems of IV-VI lasers, the inventor of the present invention proposed a IV-VI quantum well (QW) vertical cavity surface emitting laser (VCSEL) on a [111] orientated BaF2 substrates, as reported in “Infrared Applications of Semiconductors III, Symposium,” Materials Research Society Symposium Proceedings, Vol. 607, p. 181-84 (2000), the entire content of which is hereby expressly incorporated herein by reference. The VCSEL on a [111] orientated BaF2 substrate significantly improved heat dissipation and had excellent beam quality with a circular and near-diffraction-limited single-mode. The degeneracy of the L valleys was lifted with growth on the [111] orientation, and threshold carrier concentration was reduced by up to a factor of four. Pulse laser emission at 4.6 μm was observed up to 290K. Key elements of the VCSEL on a [111] orientated BaF2 substrate that contributed to its success included the BaF2/PbSrSe broadband high-reflectivity distributed Bragg reflector (DBR), the improved epitaxial material quality on the [111] orientated substrate, and the lifted degeneracy of energy-bands by the [111] QW.
The lead salt VCSELs on [111] orientated BaF2 substrates have progressed, and have obtained a 300 mW output power and a threshold density as low as 10.5 kW/cm. Above-room-temperature pulsed emission for a IV-VI material VCSEL on a [111] orientated BaF2 substrate has also been obtained. Further, for [111] orientated IV-VI QW structures, peak output power with pulsed pumping have obtained an output power of 61 Watts at 180K, 17 Watts at 250K and 4.9 Watts at room temperature. However, while these previous VCSELs on [111] orientated BaF2 substrates have proven successful in some aspects, the tuning and tuning range (as determined by cavity mode tuning) of these VCSELs have been limited. A key requirement for a laser used for optical absorption spectroscopy applications is the tunability of the center wavelength of the laser to appropriate resonances. Therefore, a need still exists for an agile continuous wave laser which is tunable over a broad portion of the IR spectrum.
While problems of material quality, efficiency, gain, etc., associated with various semiconductor structures have been generally described above with reference to laser devices, these same problems are also seen in semiconductor structures used in other electronic applications, including optoelectronic and thermoelectronic applications. Thus, there is a general need for a more efficient and effective semiconductor structure for electronic applications. It is to such a structure, and methods of making and using the same, that the present invention is directed.